1. Field of the Invention
The present invention relates to a method for locating a backscatter-based transponder.
2. Description of the Background Art
Transponders are used in radio frequency identification (RFID) systems. In these systems, unidirectional or bidirectional wireless transmission of data takes place between one or more base stations, or readers, and one or more transponders. Sensors, for example for temperature measurement, may also be integrated in the transponders. Such transponders are also referred to as remote sensors.
The transponders and their transmitting and receiving devices customarily do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply, or operating power, from the electromagnetic field emitted by the base station.
In general, a so-called backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, or a carrier signal, which the transmitting and receiving device in the transponder modulates and reflects appropriately for the data to be transmitted to the base station using a modulation method, in particular a subcarrier modulation method. A typical modulation method for this purpose are amplitude modulation or amplitude shift keying as described in Klaus Finkenzeller, RFID-Handbuch, 3rd edition, HANSER, 2002, see especially Chapter 6.2.1 Amplitude shift keying, which has been published in English by John Wiley & Sons, and as described in U.S. Pat. No. 6,046,683. Alternatively, a phase modulation can be used.
The commonly used data transmission protocols differentiate between so-called “reader-talks-first” (RTF) protocols and “tag-talks-first” (TTF) protocols. With the TTF protocol, the base station transmits an unmodulated carrier signal. When a transponder enters the communicating zone of the base station, that is, when it can detect the unmodulated carrier signal emitted by the base station, it automatically, that is, on its own transmits a signal, which can be received by the base station. The signal transmitted by the transponder can have a transponder identification, for example. In contrast thereto, with the RTF protocol, a data transmission from the transponder to the base station is initiated by sending an order, that is, a command, from the base station to the transponder, that is, the transponder does not transmit a signal and/or information automatically or on its own. To send a command, it is common for the command to be coded by the base station and the coded signal to be transmitted by modulation of the carrier signal.
With unidirectional RFID systems, information is merely transmitted from the transponder to the reader, that is, the TTF protocol is commonly used here. With bidirectional RFID systems, the information is transmitted between the transponder and the reader in both directions. Both TTF and RTF systems are used here.
In so-called pick-and-place applications, an object to be placed is taken out of a package by a gripper, for example, and then placed in a specific location, for example on a printed circuit board. If the original location of the object to be placed is unknown, such as is the case for pallets that as part of a logistics chain are set down in spots that are not precisely determined in advance, the objects to be placed must be located spatially in a first step of the pick-and-place application.
When objects such as pallets or the individual packaging units of a pallet are provided with transponders for identification, the transponder can be used for both identifying and locating the goods. In this context, identification is typically accomplished by entering a goods identifier in a memory area of the transponder. The process of spatial location can take place using a locating signal transmitted by the transponder. In this context, a robot can use the locating signal as a guide beam. A method for locating is described in U.S. Pat. No. 6,046,683.
In U.S. Pat. No. 6,046,683, the transmission of the locating signal is initiated by the base station on the basis of the RTF protocol by contacting, that is, isolating or singling out an initially unknown transponder from a plurality of transponders, in order to enable, directly or indirectly, the transponder to transmit the locating signal. In other words, the base station must continuously send information queries in order to detect transponders that are newly entering the communicating zone. This so-called polling causes an interference spectrum.
If, in addition to the locating function, that is, with an active locating signal, the transponder is to be able to receive data, the duration of the locating signal must be time-limited with transponder systems that feature amplitude modulation of the backscattered signal, or else the locating signal sent by the transponder must be interrupted, or suppressed, at certain time intervals to enable the transponder to receive data.
This is based on that data transmission from the base station to a transponder is customarily accomplished with the use of amplitude modulation of the carrier signal transmitted by the base station in the form of sequential field gaps or field attenuations of the carrier signal, which are also called notches. Such notches are detected in the transponder by means of receiver signal strength indicator (RSSI) circuits.
When the transponder's locating signal is generated through amplitude modulation and backscattering of the carrier signal by the transponder, the real part of an input impedance of the transponder is changed, see Finkenzeller, chapter 4.2.5.4, effective aperture and scatter aperture. The change in the real part of the input impedance leads to a matching or mismatch of the transponder's input impedance to an impedance of a transponder antenna, which changes the reflection properties and receiving properties of the transponder antenna, with the result that the transponder or its antenna reflects or receives more or less power. In this context, the modulation index is customarily relatively large in order to achieve correspondingly high field strength differences between an absorption state, that is an adapted state, and a reflector state, that is an error adapted state, that is to effect comparatively large signal-to-noise ratios of the backscattered signal.
The transponder's received level strongly decreases as the distance increases between the base station and the transponder so that, in contrast to 125 kHz RFID systems, the received level in the transponder must be amplified. Thereby, the self caused fluctuations of the input amplitude of the received carrier signal that are caused by the transponder itself, an that arise during the generation of the locating signal through amplitude modulation and backscattering of the received carrier signal, can only be distinguished with great effort from the notch signals generated by the base station, which likewise result in a change in the amplitude of the received carrier signal. This is especially the case when the field attenuation for notch generation (modulation index) is kept small in order to increase the range. Additional complex input filters are then required in the transponder in order to differentiate. In order to achieve differentiation by means of filters, the sidebands of the received and backscattered signals must additionally be relatively widely separated. This is often impossible to achieve, since applicable standards and/or regulations would otherwise be violated.
In summary, this means, as described in detail above, a communication between transponder and base station can take place due to the amplitude modulation of the transponder, which means that the transponder is unable to receive modulation signals from the base station while the locating signal is being sent. Consequently, the duration of the locating signal has to be time-limited, or else the locating signal emitted by the transponder must be interrupted, or suppressed, at certain time intervals to enable the transponder to receive data.
However, certain applications require transponders, which generate a locating signal that is continuous and not time-limited, and which simultaneously can be polled by a base station, for example, to inquire about their identification, or to program them, once they have been recognized by the base station by virtue of the locating signal transmitted by the transponders.
In the locating method described in U.S. Pat. No. 6,046,683, the transmission of the locating signal by the base station is initiated by an appropriate polling command sent by the base station to the transponder. Because the method described in U.S. Pat. No. 6,046,683 is based on the RTF protocol, in order to detect whether or not a transponder is within their polling range, the base station must continuously send polling commands in order to get a response from transponders that are newly entering their polling zones. However, such a permanent transmission of information by the base station generates an interference spectrum, which is frequently undesirable. Furthermore, the polling command has to be sent at a high frequency so that a transponder newly entering the communication range of the base station can be detected with little delay.